An interesting problem in quantum optics is the interaction between a single atom and a single quantized mode of an electromagnetic field. This problem has an exact solution if irreversible processes which give rise to damping, such as atomic spontaneous emission and decay of the field mode, are negligible. If they are negligible, the atom and the field mode exchange energy in a manner characteristic with that of coupled oscillators. The rate of energy exchange, determined by the atom-field coupling strength, can be appreciable even for a vacuum field. This coupling has been observed in the line shape splitting for a weakly-excited absorbing atom.
Atom-cavity interaction is often referred to as cavity quantum electrodynamics (hereinafter "cavity QED"). The single-atom maser or micromaser, one system for studying cavity QED was developed in 1985, and various aspects of cavity QED such as quantum collapse and revival and nonclassical atom statistics have been studied.
In a single-atom micromaser, a beam of excited atoms is injected into a high-Q microwave resonator. The stream of atoms is injected at a controlled flux rate so that not more than one atom is present inside the cavity at any time. A photon released into the cavity by each atom resonates in the cavity for a long enough time period to interact with subsequent atoms in the stream. The coupling between the single atom and the photon field strengthens as the field builds up, and can eventually become steady-state. Single-atom masers operate in the microwave regime. The photons released by the interaction are of low energy and momentum and therefore are difficult to detect and monitor. The resonators are made with walls composed of low temperature superconductive material and are thus difficult to maintain and expensive to fabricate.